1. FIELD OF THE INVENTION
This invention relates generally to radiation reflectors, commonly called dishes, which are used in combination with a radiation transducer, such as a radiation emitter or detector, situated at a focus of the reflector to project or receive a radiation beam along a path parallel to the principal axis of the dish. The invention relates more particularly to a collapsible and deployable offset dish of this kind whose focus, and the radiation transducer used with the reflector, are situated outside the dish beam path.
2. PRIOR ART
Radiation reflectors or dishes of the type to which this invention pertains are utilized over a wide range of the electro-magnetic radiation spectrum and in a variety of reflector-type radiation transmitting and receiving devices. Such radiation transmitting and receiving devices are collectively referred to herein simply as radiation devices. Examples of such radiation devices are parabolic dish antennas, solar concentrator collectors, high energy beam devices and the like.
Radiation devices of the character described are commonly characterized by a radiation reflector dish and a radiation transducer situated in front of the dish. This transducer may be either or both a radiation emitter, which emits radiation toward the dish that is reflected outwardly along the principal axis of the device, and/or a radiation detector, which receives radiation incident on the dish from a remote source that is reflected to the transducer. In a parabolic dish antenna, for example, the transducer may be either or both a radiation emitter called a feed or a radiation detector. In a solar concentrator - collector, the transducer may be a solar energy converter, such as a solar cell. In a high energy beam device, the transducer may be a high energy radiation source. Other high energy beam devices may have a reflector only for collecting and focusing high energy beams in space.
The present invention will be disclosed primarily in connection with a parabolic dish antenna. It will become evident as the description proceeds, however, that the principles of the invention may be utilized in other radiation devices of the class described.
The construction, operation, and characteristics of a conventional parabolic dish antenna are well-known and understood. Accordingly, it is necessary to describe the conventional antenna only in sufficient detail to enable a full and complete understanding of the invention.
A conventional parabolic dish antenna has a parabolic or paraboloidal reflector dish whose reflecting surface conforms substantially to a parabolic surface of revolution, that is a paraboloidal surface, generated about an axis of the dish called its principal axis. The surface has a focus located on this axis. Situated at the focus is the antenna transducer, which may be either or both a radiation emitter or feed and/or detector. In the antenna art, this transducer is commonly referred to as the primary aperture of the antenna. The antenna dish is referred to as the secondary aperture of the antenna.
In a transmitting parabolic dish antenna, radiation is emitted from its primary aperture or feed transducer toward its secondary aperture or dish and is reflected from the latter in the form of a beam parallel to the principal axis of the dish. In a receiving antenna, a beam of incoming radiation incident on the secondary aperture or dish parallel to its principal axis is reflected to the primary aperture or detection transducer. In the following description, the paths of these outgoing and incoming beams are referred to as the beam paths of the secondary aperture or dish.
Two different types of parabolic dish antennas, referred to herein as symmetrical and offset antennas, have been devised. The secondary aperture of a symmetrical parabolic dish antenna is normally a circular dish that is symmetrical about its principal axis so that this axis passes through the center of the dish. The primary aperture of the symmetrical antenna is situated in front of the secondary aperture and is, therefore, located in the secondary aperture beam path.
A transmitting symmetrical parabolic dish antenna has two disadvantages: (1) the beam reflected from the secondary aperture or dish impinges the antenna feed and alters its input impedance; (2) the antenna feed and its support obstructs and distorts the reflected beam. In a receiving symmetrical parabolic dish antenna, its detector is situated in the incoming beam path and, hence, alters and distorts the incoming beam.
An offset parabolic dish antenna eliminates these disadvantages. To this end, the secondary aperture of such an antenna comprises a paraboloidal dish which is only an offset portion of the symmetrical antenna dish, that is, a portion of the symmetrical dish offset from the principal axis of the dish. In such an offset antenna, therefore, the principal axis and focus, and hence also the primary aperture, are offset to one side of the secondary aperture so that the primary aperture is located outside the secondary aperture beam path.
Many applications of parabolic dish antennas permit the use of fixed dishes which are permanently fixed in their paraboloidal operating configuration. Other applications of such antennas, notably space applications, require the capability of collapsing the antenna to a compact configuration for storage and deploying the antenna to its paraboloidal operating configuration. The present invention is particularly concerned with such collapsible and deployable parabolic dish antennas.
The prior art is replete with a variety of collapsible/ deployable parabolic or paraboloidal antenna dishes. Examples of collapsible/deployable symmetrical paraboloidal antenna dishes are described in the following patents:
______________________________________ 2,572,430 3,617,113 2,806,134 3,635,547 3,064,534 3,699,756 3,176,303 3,707,720 3,286,270 3,715,760 3,360,798 3,717,879 3,397,399 4,030,102 3,521,290 4,314,253 3,541,569 4,315,265 3,576,566 4,352,113 ______________________________________
U.S. Pat. Nos. 4,030,103 and 4,498,087 disclose parabolic dish antennas with a collapsible/deployable off-center paraboloidal antenna dish.
Some of the collapsible/deployable paraboloidal antenna dishes disclosed in the above patents utilize reflecting surfaces of wire mesh or the like. These antennas have functioned quite satisfactorily up to the present time because of their compatibility with the wavelengths that have been used in the past and will continue to function satisfactorily for those applications that involve such wavelengths.
There is, however, an ever increasing use of shorter and shorter wavelengths of the electromagnetic spectrum, as well as an increasing interest in collecting and focusing light waves and other short wavelength energy in space. Reflector dishes for these shorter wavelengths and light waves must satisfy stringent requirements of smoothness and contour in order to minimize scattering and enhance gain. These requirements have resulted in increasing usage of reflectors whose reflecting surfaces are solid, as contrasted to wire mesh for example, and rigid, as contrasted to a metal coated plastic membrane.
Many of the patents listed above disclose collapsible/ deployable parabolic dishes which provide such a relatively solid and rigid paraboloidal reflecting surface when deployed. These dishes commonly comprise relatively rigid solid panels which are foldable and unfoldable between contracted and extended positions. When the panels are contracted, the dish is collapsed to a compact storage configuration. When the panels are extended, the dish is expanded to its deployed configuration, wherein the dish provides a relatively solid and rigid paraboloidal reflecting surface. These solid rigid collapsible and deployable dishes, however, are all symmetrical dishes which suffer from the disadvantages discussed earlier.
The only listed patents disclosing parabolic dishes which are offset parabolic dishes and, hence, are not subject to such disadvantages are U.S. Pat. Nos. 4,030,103 and 4,498,087. These offset parabolic dishes, however, have a wire mesh reflecting surface and, therefore, are not suitable for the shorter and shorter wavelengths and light waves which are now in use or contemplated for future use. Accordingly, there is a need for a solid rigid offset parabolic dish.